Recovery of phosphorus from incinerated sewage sludge ash by combined two-step extraction and selective precipitation

Abstract

The recovery of phosphorus (P) from acid extracts of incinerated sewage sludge ash (ISSA) is invariably complicated by the co-dissolution of metal(loid)s. An extra purification treatment step is thus needed, which reduces the efficiency of P recovery. This study developed a two-step extraction method which can produce highly purified P-extract where ISSA is treated first with ethylenediaminetetraacetatic acid (EDTA) and then with sulphuric acid. The recovery conditions of this two-step extraction method were optimized (pre-extraction by 0.02 mol/L of EDTA at a liquid to solid ratio (L/kg) of 20:1 during 120 min, then extraction by 0.2 mol/L sulphuric and at a liquid to solid ratio of 20:1 during 120 min). Compared with direct extraction by sulphuric acid (namely, single-step leaching), the two-step extraction method dissolved 2.34 mmol/kg ISSA less of P, but with 5.16 mmol/kg ISSA less of metal(loid) contamination.Furthermore, the potential for co-precipitation of P and the metal(loid) contaminates in the extracts (both single-step extraction and the two-step extraction methods) was examined from pH 2 to 12 (adjusted by addition of NaOH and Ca(OH)2). By applying Ca(OH)2 and at the optimal pH of 4, the two-step extraction method could significantly increase Ca-P in the precipitate and notably decreased the metal(loid) contaminants by 50% compared to the single-step method.